There is an increasing population of electrical and electronic equipment utilizing alternating current line rectifiers, especially office computers, motor speed controllers and other equipment which draw nonlinear currents. A concern in the power engineering industry focuses on the disruptive harmonic effects of voltage transients and spikes caused by those electronic or nonlinear loads. Many times, the equipment which is causing those harmonics is, at the same time, sensitive to those harmonics that similar devices are creating. In addition to the harmonics damaging the device producing them, those harmonics may also adversely affect other pieces of electrical equipment connected to the power line and may even damage the power generation and distribution system itself.
Nonlinear loads may affect two types of power systems: single-phase, most common in office settings, and three-phase, most common in industrial applications. Harmonic induced problems frequently manifest themselves in improper operation, malfunction, shutdown of the equipment, or premature equipment failures. In three-phase, four wire systems, some harmonic currents, for example, the third harmonic, do not null at the neutral conductor, but rather sum, which may cause over-loading of that neutral conductor. The inherent danger in such overloading is the risk of damage or fire, as there is no circuit breaker on the neutral to limit the current flow.
Known techniques to improve the quality of the power system voltage waveforms are largely ineffective in correcting that problem. One common way to address the harmonics problem is to attempt to restrict harmonic currents by use of passive filters or harmonic traps. Those filters are tuned to a specific frequency, normally the highest amplitude harmonic. Filters operate by "notching out" portions of the disturbance. Such filters may absorb some of the harmonic energy, however, the dissipated energy is wasted as nonproductive heat loss. Further, such absorption is limited to a specific frequency and thus, voltage spikes or dips which occur at frequencies other than the specific frequency of the filter are not corrected nor are they correctable without the addition of an additional filter.
Generic "line clipping devices" do not restore minor voltage dips or harmonic distortion and can not clamp or reduce any electrical disturbances that occur below the peak amplitude of the sine wave. With respect to three-phase systems, to upgrade the present electrical distribution system to reduce the effective power source impedance by substituting larger neutral and phase conductors is both costly and often an impractical solution.
Applicant's copending U.S. patent application Ser. No. 08/089,226, filed Jul. 8, 1993, (the "226 application"), which is hereby incorporated by reference, addresses those problems caused by non-linear loads. The invention claimed in the '226 application substantially reduces or eliminates the harmonic effects which nonlinear loads and switched power loads may create on a power distribution system, thereby permitting the distribution system to provide clean, quality power to other sensitive equipment. That invention provides a local, low impedance alternating current source of quality power to satisfy the requirements of sensitive load equipment which may be connected to its output.
The invention as described in the '226 application is effective in making non-linear load currents appear linear to the power source. However, drawing continuous current through a device constructed according to the teachings of the '226 application may cause a voltage droop and slight phase shift across the series inductor used to isolate the power source from the storage module. While non-linear loads are tolerant of those effects, linear loads may not be as tolerant and may fail to operate properly if subjected to such undervoltage conditions. In applications comprising multiple loads, for example, in an office or laboratory environment, the load on a power system is typically comprised of a mixture of both linear and non-linear devices. Thus, at least a portion of the equipment is drawing linear current and is subject to such undervoltage conditions.
Accordingly, a further improvement is needed beyond that of the '226 application such that linear loads connected to such power systems continue to operate effectively.
It is an object of the present invention to substantially reduce or eliminate the harmonic effects which non-linear loads and switched power loads may create on a power distribution system similar to the effects of the invention described in the '226 application without affecting the operation of either linear or non-linear loads. It is a feature of the present invention to provide stored energy in a parallel resonant circuit to smooth the effects which Nth-order harmonics and voltage spikes or dips may have on that power distribution system. It is a further feature of the present invention to provide an in-line inductance to further isolate the power source from the effects of non-linear loading. It is yet another feature of the present invention to provide an in-line capacitance to prevent voltage droop or phase shifting of the power signal. It is an advantage of the present invention that when harmonics and/or transient voltage disturbances do occur on the power line, the effects of such disturbances will not impact the equipment attached thereto as the power line waveform will be instantaneously smoothed while maintaining acceptable voltage levels. A further advantage of the present invention is that a device embodying the present invention may be advantageously applied to any type of sinusoidal, fixed frequency, alternating current power system.